This invention relates to solar energy collection, and in particular to an arrangement for driving a number of rows of solar panels to track the motion of the sun relative to the earth. The invention is more particularly directed to improvements in efficiency and reliability in the tracker arrangement for rocking, or rotating, a group or array of rows of solar panels. The invention applies to solar collectors in which the panels are arrays of photovoltaic cells for generating electrical power, but the same principles can be applied also to arrangements for solar heating, for example.
Photovoltaic arrays are used for a variety of purposes, including as a utility interactive power system, as a power supply for a remote or unmanned site, a cellular phone switch-site power supply, or a village power supply. These arrays can have a capacity from a few kilowatts to a hundred kilowatts or more, and can be installed wherever there is a reasonably flat area with exposure to the sun for significant portions of the day.
In general terms, these systems have their photovoltaic panels in the form of rows supported on a torque tube that serves as an axis. A tracker drive system rotates or rocks the rows to keep the panels as square to the sun as possible. Usually, the rows are arranged with their axes disposed in a north-south direction, and the trackers gradually rotate the rows of panels throughout the day from an east-facing direction in the morning to a west-facing direction in the afternoon. The rows of panels are brought back to the east-facing orientation for the next day.
One solar collector arrangement of this type is shown in Barker et al. U.S. Pat. No. 5,228,924. There, each row of panels is affixed to a horizontal pivot shaft that is supported on two or more support piers on which the pivot shaft is journalled. A drive mechanism is mounted on one of the piers, and pushes against the solar panel at some point that is displaced from the shaft. In that case, the drive is of the screw type, and as a drive motor rotates, a shaft retracts or extends to rotate the row of panels in one direction or the other. In this arrangement, each row of panels has its own respective drive mechanism, and so these all have to be synchronized to follow the sun together. With a pier-mounted drive, it is difficult or impossible to use a single driver to move more than one row of solar panels.
One problem with this arrangement, i.e., with the pier mounted drive mechanisms, is their difficulty in dealing with the enormous torque loads that can result from winds and other weather phenomena. There is great complexity in the drive and pier fittings because of this. Moreover, the pier itself has to be strengthened to carry the drive forces to the foundation in which it is mounted. The pier also is subjected to bending loads imposed by the pier-mounted drive. Because the pivot shaft and drive are both mounted on the pier, it is not possible to accommodate a large torque arm or lever arm, and so this imposes a large load factor onto the drive mechanism.
Conventional trackers of this type typically employ square steel tubes as beams that span between piers because, given the presence of wind-generated loads, this shape better carries the torsion loads between the panels and the drive. However, in order to join successive sections of a square torque tube or beam, special connectors are required, which add complexity and expense to the construction. Also, these square torque tubes have to be accommodated both rotationally, radially, and axially, where they are journalled in the supporting piers, because of the weight and wind loading of the panels, and because of thermal expansion of the torque tube. Over time, metal-to-metal contact between the torque tube and the journal will destroy corrosion-protective finishes. However, the bearings or pivot journals for the conventional tracker systems do not address these problems.